Some phosphate minerals contain uranium in a higher concentration than most of the constituents of the terrestrial layer (from a few tenths to a few hundred parts per million compared to about 3 parts per million).
During production of phosphoric acid by the wet process method, the phosphate mineral or rock is reacted with a mineral acid, generally sulfuric acid, and almost all of the uranium passes into the phosphoric acid in the form of U+4 and U+6 ions. A phosphoric acid produced from a Florida mineral: can have the following composition: P.sub.2 O.sub.5 : 29.37%--SO.sub.4 : 2.77%--F: 2.15%--different materials in solution: 0.18%--Fe.sub.2 O.sub.3 : 0.86--Al.sub.2 O.sub.3 : 0.81%--MgO: 0.35%--U.sub.3 O.sub.8 : 1.66 parts per million (ppm).
With this mineral, it is possible to produce about 500 g uranium oxide U.sub.3 O.sub.8 per ton of P.sub.2 O.sub.5, which corresponds to about 150 tons a year of uranium oxide in an industrial unit producing 1,000 tons per day of P.sub.2 O.sub.5.
Processes have been used for purifying phosphoric acid and at the same time recovering the uranium. The processes that have been developed commercially are the liquid-liquid extraction processes. The liquid-liquid extraction processes can be classified into 3 groups:
(1) Extraction of uranium, reduced to the U+4 state by octyl phenyl esters of orthophosphoric acid, (MOPPA-DOPPA) or octyl pyrophosphoric acid (OPPA) in a solution in kerosene.
(2) Extraction of uranium, oxidized in the U+6 state, by a solvent comprising a mixture of bis(2 ethylhexyl)phosphoric acid (D2EHPA) and trioctyl phosphine oxide (TOPO).
(3) A mixed processes using OPPA and a D2EHPA and TOPO mixture (disclosed in U.S. Pat. No. 3,835,214).
Other processes have been disclosed, that use solutions of more complex reagents such as the diphosphonic acid esters described in French patent application No. 2,460,958.
These processes are objectionable because of a low extraction efficiency requiring multiple extraction stages coupled with a further complication involving a solvent regeneration cycle requiring many steps. In addition, contacting the acid with the extractive solvents causes a considerable loss of solvents and contamination of the acid produced.